Many plants synthesize and accumulate proline in response to osmotic stress. Drought and high salinity are the most important environmental factors that cause osmotic stress and impact negatively on plant growth and crop productivity. The role of proline biosynthesis in osmoregulation in bacteria is well established in, for example, Csonka, Microbiol. Rev. 53, 1989, pp. 121-147. The proline biosynthesis route in plants is not as well known but is thought to follow the biosynthesis route in bacteria. In Escherchia coli, the synthesis starts with the phosphorylation of glutamate by gamma-glutamyl kinase, hereinafter gamma-GK, which is encoded by the proB gene. The gamma-glutamyl phosphate thus formed is reduced to glutamic-gamma-semialdehyde, hereinafter GSA, by GSA dehydrogenase (GSD) which is encoded by the proA gene. The GSA thus produced spontaneously cyclizes to delta.sup.1 -pyrroline-5-carboxylate (P5C) which is delta.sup.1 -pyrroline-5-carboxylate reductase, hereinafter P5CR, which is encoded by the proC gene, to thereby form proline.
With the exception of P5CR which has been recently characterized (Krueger et al., Plant Physiol., 82, 1986, pp. 890-903; Rayapati et al., Plant Physiol., 91, 1989, pp. 581-586; Delauney and Verma, Mol. Gen. Genet., 221, 1990, pp. 299-305; Verbruggen et al., Abstract, Third International Congress, International Society for Plant Molecular Biology, Tucson, Ariz., Oct. 6-11, 1991), however, little is known about the other enzymes in this pathway in plants.
Therefore, it would be desirable to use genetic engineering of the proline production pathway in plants to counter osmotic stress to alter the level of a known osmoprotectant to thereby lead to a significant enhancement of crop performance under conditions of salt and drought stress.